The invention relates to the improvement of metalloceramic materials, and more particularly, it relates to sintered metalloceramic current-conducting materials and methods for preparing them.
The invention can advantageously be used in manufacturing powerful resistors used in radio engineering, load resistors for industrial use, and electric elements for household and industrial heating appliances ensuring working temperatures up to 700.degree.C.
Known in the prior art is a metalloceramic current-conducting material and a method for preparing it according to U.S. Pat. No. 3,309,643. The metalloceramic material is prepared on the basis of a powdered nickel-chromium alloy as a current-conducting component, and a refractory filler material such as powdered aluminium oxide. The nickel-chromium powder is taken in an amount of 20 percent by weight, and the amount of aluminium oxide powder is 80 percent by weight, in other words, the metalloceramic material contains a great amount of the refractory filler material, and a small amount of the current-conducting component.
This ratio of the current-conducting component to the refractory filler material is necessary to give a high specific resistance to the current-conducting material which is used for the manufacture of volumetric electric heating elements. However, if the amount of the current-conducting component in the material is small, the distribution of specific electric resistance throughout the bulk of the metalloceramic material is very uneven, which is due to the special sensitivity of such materials to natural heterogeneity of the composition. The necessity of taking the components in the specified ratio to prepare a current-conducting material having a high specific resistance is due to the significant difference in the electric conductivity of the nickel-chromium alloy and the aluminium oxide, and also low resistance of metallic contacts between the nickel-chromium alloy particles. The low resistance of the metallic contacts between the nickel-chromium alloy particles is explained by the fact that the particles of the nickel-chromium alloy are devoid of oxide films that otherwise might insulate them from one another and thus increase the electric resistance of the contacts between them.
The method for preparing the known metalloceramic current-conducting material resides in that one part of the alloy, consisting of about 80 percent of nickel and 20 percent of chromium, is mixed with four parts of aluminium oxide powder for 24 hours, after which the mixture is compacted under a pressure of about 10 tons/sq.cm, and the pressed mixture is sintered at a temperature of 1500.degree.C in hydrogen or another reducing medium for 24 hours.
The known metalloceramic current-conducting material is used as electric elements for household heaters. The preparation of this metalloceramic current-conducting material is however expensive, since the pressing of powders such as aluminium oxide is difficult due to their low plasticity; as a result binders are required and the press-moulds are quickly worn out.
The known method incorporates sintering of the powders of the nickel-chromium alloy and aluminium oxide, which requires heating to temperatures of about 1500.degree.C for a long time to sinter the refractory filler material, the basic component of the known metalloceramic current-conducting material. Moreover, the process for preparing the known metalloceramic current-conducting material is made even more complicated and expensive because it is necessary to use a reducing medium for the sintering process, which is fraught with danger of explosion.
Known also is another metalloceramic current-conducting material and the method for preparing it according to U.S. Pat. No. 3,184,835. The known metalloceramic current-conducting material is prepared on the basis of crushed solid solutions of a readily oxidizable metal such as - beryllium, magnesium, or aluminium, taken in an amount from 0.05 to 0.69 percent by weight, in a matrix of copper or silver.
Oxides of aluminium, magnesium, or beryllium, are refractory insulating components of the metalloceramic current-conducting material, but they are contained in an amount approximately from 0.1 to 1.7 percent by weight. This amount of the insulating refractory material cannot substantially change the specific electric resistance of materials having a low specific resistance such as copper, silver, or their alloys.
The method for preparing the known metalloceramic current-conducting material resides in that solid solutions of a readily oxidizable metal, such as beryllium, magnesium, or aluminium, taken in an amount from 0.05 to 0.69 percent by weight in copper or silver are crushed and oxidized by heating in an oxidizing medium, after which the internally oxidized powder undergoes hot extrusion. During the internal oxidation, minute particles of the oxide of the dissolved metal (beryllium, magnesium, or aluminium) are formed inside the copper or silver matrix.
The formation of the finely dispersed phase of solid oxides of beryllium, magnesium or aluminium improves significantly the mechanical properties of the metalloceramic material without appreciably changing its electrical resistance. This material cannot be used, for example, to manufacture resistors, load resistors, or electric elements for household or industrial heaters due to its low specific electric resistance.
Known in the prior art also is a current-conducting metalloceramic material and the method for preparing it according to Japanese Pat. No. 40-4451. The metalloceramic material is prepared on the basis of ground steel containing from 0.3 to 5 percent by weight of aluminium.
The formation of fine particles of aluminium oxide inside the steel matrix somewhat increases its specific resistance but due to the low content of aluminium oxide in the metalloceramic material, the specific resistance remains low, which makes it impossible to use the material in the manufacture of resistors, load resistors, or electric elements for household and industrial heaters.
The method for preparing this metalloceramic current-conducting material consists in the following. Steel powder containing from 0.3 to 5 percent by weight of aluminium, is dispersed, the powder is oxidized in air at a temperature of 600.degree.C, during which the aluminium, that is present in solid solution in the steel, is oxidized to aluminium oxide, then the steel powder is reduced in a reducing medium, pressed into the required shapes and sintered in a reducing medium. The method increases the refractoriness and strength of the steel without changing substantially its electrical properties. The method is very similar to that covered by U.S. Pat. No. 3,184,835. Both methods fail to prepare metalloceramic current-conducting materials having a high specific resistance and the required refractoriness.